The presence of and transport through cell walls of metallic ions has long been recognized as having considerable physiological importance. Various methods have been developed for carrying out such measurements. The principal methods used heretofore have been the use of radioactive isotopic tracers and the use of ion selective electrodes. The use of radioactive tracers carries with it the problem of radioactive waste and the necessity of a certain amount of destructive testing of sacrificial aliquots as well as extensive subsequent analytical workup for each point in a transport study.
While ion-selective electrodes are of considerable significance they can not be utilized to study small cells or organelles because of size limitations.
Until recently, nuclear magnetic resonance technology could not be utilized in such studies since the time response of the available machines was not sufficiently rapid. New machines have made possible response time of the order of tens of milliseconds and therefore the use of NMR techniques for kinetic studies of ionic transport in cells has become feasable. The use of NMR technology requires the provision of reagents to differentiate between ions on two sides of the cell wall. Such reagents cause a shift in the NMR spectrum and are known as shift reagents. Heretofore, sufficiently powerful shift reagents for the shifting of the spectrum of alkali and alkaline earth metal ions in an aqueous environment have not been available.